The invention relates to optical transmission systems and more particularly relates to dealing with so-called polarization mode dispersion in such systems.
Polarization Mode Dispersion (PMD) occurs in an optical fiber as a result of a small residual birefringence that is introduced in the fiber core by asymmetric internal stress as well as random polarization coupling due to external forces acting upon the fiber. It is well-known that PMD may severely impair the transmission of a signal in an optical fiber network. This is especially the case in modem digital lightwave systems that operate at bit rates of at least 10 Gb/s per transmitted wavelength channel.
U.S. Pat. No. 5,930,414, which issued Jul. 27, 1999 to D. A. Fishman et al, discloses different apparatus for mitigating signal impairments due to PMD. Such apparatus employs a variable optical birefringence element, which introduces a differential optical time delay between at least two selectable mutually orthogonal polarization states, i.e., the xe2x80x9cPrincipal States of Polarizationxe2x80x9d (PSP) in the transmission fiber. An optical distortion analyzer coupled to the output of the variable birefringence element generates a control signal for that element.
The compensation apparatus shown in FIG. 4 of the referenced ""414 patent generates a continuously variable Differential Group Delay (DGD) between two principal states of polarization to compensate for first-order PMD in the transmission fiber. One difference between this arrangement and the time-delay line shown in FIG. 1 of the referenced ""414 patent is that the PSPs of the variable birefringence element in the apparatus of FIG. 4 of the ""414 patent is frequency dependent. Such frequency dependence (which is an important aspect of a so-called second-order PMD effect) may lead to excessive distortion in the optical signal, since the first-order PMD in the fiber can only be compensated for over a certain optical bandwidth. Moreover, the PMD may even increase in certain parts of the optical spectrum.
Signal distortion caused by the second-order PMD effect in conventional non-return-to-zero (NRZ) and return-to-zero (RZ) digital optical signals may be observed in the electrical spectrum of the received optical signal as a narrowband xe2x80x9ctonexe2x80x9d of spectral energy at the certain frequencies related to the signal bit rate, e.g., 10 GHz for NRZ and 20 GHz for RZ signals.
We decrease substantially the distortion caused by second-order PMD effects in a variable birefringence element of PMD compensation apparatus by applying a control signal to the variable birefringence element such that the optical signal experiences minimal second-order PMD. The present invention provides a distortion analyzer that can be used for generating such control signal by, e.g., processing particular spectral components of the optical signal. For example, the control signal may be generated by adding a first signal corresponding to the entire spectrum of the optical signal to a second signal corresponding to a low-frequency component of the optical signal and then subtracting from the result of addition a third signal corresponding to a specific frequency band of the optical signal. The resulting control signal, Vf, may then be supplied to the polarization controllers in the variable birefringence element to reduce the level of the distortion that occurs as a result of the aforementioned second-order effect. The generated Vf, more particularly, causes the PMD compensator to operate at a point where the overall second-order distortions in both the transmission fiber and PMD compensator are minimal.
These and other aspects of our invention are set forth in the following detailed description, corresponding drawings and ensuing claims.